1. Field of the Invention
The present invention relates to a complex including indocyanine green and a gelatin derivative, a contrast agent for photoimaging using the same, and a method for producing the complex.
2. Description of the Related Art
A photoacoustic tomography (hereinafter may be referred to as PAT) apparatus is known as one of apparatuses for visualizing in-vivo information. In the measurement using a PAT apparatus, a tomographic image can be obtained by measuring the intensity and the time of generation of a photoacoustic signal emitted from a substance (optical absorber) that absorbs the light in an object to be measured when the object to be measured is irradiated with a light and computing a distribution of the substance in the object to be measured.
Here, any substance can be used as an optical absorber, so long as the substance absorbs a light and emits an acoustic wave in a living body. For example, a blood vessel, a malignant tumor, or the like in the human body can be used as an optical absorber. In addition, a molecular probe such as indocyanine green (hereinafter may be referred to as ICG) can be introduced into the body and used as a contrast agent. ICG is a safe substance that has been approved to be taken into the body. Since ICG sufficiently absorbs a light in the near-infrared wavelength range, which has little influence on the irradiated human body and is highly permeable in a living body, ICG can be suitably used as a contrast agent in PAT.
Furthermore, ICG emits fluorescence when excited by a light in the near-infrared wavelength range. ICG can also be used as a contrast agent for fluorescent imaging by utilizing this characteristic.
Meanwhile, there is such a problem that it is difficult to accumulate ICG ingested as a contrast agent at a site for measurement because ICG is easily reacted with water and degraded.
As a method for solving this problem, a technique to entrap ICG in a particle at a high concentration has been developed. Journal of Photochemistry and Photobiology B: Biology, 74 (2004) 29-38 (hereinafter expressed as Saxena et al.) discloses a poly(lactide-co-glycolide) (hereinafter may be referred to as PLGA) particle containing ICG that is obtained by an emulsification solvent diffusion method using polyvinyl alcohol (PVA) as a surfactant. Furthermore, U.S. Patent Publication No. 2009/087479 specification (hereinafter may be expressed as Lau et al.) discloses a liposome containing ICG and a gelatin.